When CO2 is injected in deep saline aquifers on the scale of gigatonnes, pressure buildup in the aquifer during injection will be a critical issue. Because fracturing, fault activation and leakage of brine along pathways such as abandoned wells require a threshold pressure (Nicot et al., 2009), operators and regulators will be concerned with a critical contour of overpressure (CoP). The extent of this contour varies depending on the target aquifer properties (porosity, permeability etc.) and the geology (presence of faults, abandoned wells etc.). The extent also depends on relative permeability, and from the three-region injection model (Burton et al., 2008), we derive analytical expressions for a specific contour of overpressure at any given time. The risk of pressure-induced leakage from the aquifer can therefore be understood in terms of phase mobilities and speeds of saturation fronts. This provides a quick tool for estimating pressure profiles.
Seven different relative permeability curves (Bennion and Bachu, 2005) and their effect on the CoPs in each of the three regions have been studied. The relative permeability curve which gives the maximum two-phase region mobility (MBL) gives the lowest pressure buildup (specific CoP is closest to the injector). Thus characterizing relative permeability will be an important consideration for the practical implementation of CO2 storage projects. For smaller values of critical CoP which lie in the brine region, the location of the critical CoP, and hence the risk due to pressure buildup, are time-invariant and independent of relative permeability. This result significantly reduces the uncertainty in predicting these contours of overpressure.
Geologic sequestration of CO2 is widely regarded as one of the viable options for GHG mitigation. Because sequestration must be conducted at a very large scale, the safety and effectiveness of storage schemes will be important. To date, regulatory frameworks have focused on risks associated with the extent of the CO2 plume. But injection of such large volumes of CO2 into deep saline aquifers over a time span of a few decades also leads to significant pressure buildup. This "pressure plume" extends much farther than the CO2 plume. The risks associated with excessive overpressure in the aquifer include mechanical damage to the storage formation, fracturing the seal of the storage formation, opening faults or fractures, and displacement of brine into underground sources of drinking water (USDW).
Each of these phenomena involves a threshold pressure (see Nicot et al. (2009) for analysis of brine displacement through abandoned wells). Thus a convenient proxy for these risks is the contour of critical overpressure (CoP). The critical overpressure is the minimum increment in aquifer pore pressure that would cause any of the negative impacts mentioned above (brine leakage, mechanical damage). The CoP thus depends strongly on properties of the target aquifer. For this study, we illustrate the behavior with several arbitrary values of critical overpressure.